With the rapid development of silicon-based CMOS technology, performance and production efficiency of silicon-based CMOS circuit products have been substantially improved, and production costs of CMOS image sensor have been greatly reduced. However, production costs of CMOS image sensor can still not fully meet the market demand. As for CMOS image sensors, the current largest wafer for silicon-based CMOS process is 12 inches, while a 12-inch production line is still relatively high in production costs. 8-inch production line is relatively low in cost, however production costs of which are still too high for the market demand.
The cost of Thin Film Transistor (TFT) image sensor can be lowered further due to substrates in production lines of TFT process are relatively large, like 1 m×1 m, or even up to 3 m×3 m or so. The larger the substrate, the more individual products that can be cut, and then the lower the final cost of a single product.
In the field of image sensors, in order to reduce cost and size of a sensor, an optical signal image is usually focused through a focusing lens to become a small image and then a photoelectric conversion is performed, which is a method for image capture of the traditional CMOS sensor and CCD sensor and so on.
However, in some fields, in order to improve an image performance and optimize a structural design, imaging can not be performed using a focus lens but be performed with a ratio of 1 to 1, such as imaging in an X-ray flat-panel image sensor. The “flat-panel” herein means that an incident light signal has not been focused, but been directly imaged with the ratio of 1 to 1.
The commonly used X-ray flat-panel image sensors usually have a large pixel area, which may reach 17 inches by 17 inches. If the CMOS image sensors are applied to manufacture the X-ray flat-panel image sensors, a plenty of small sensors will be required to be joint together, then the cost will be very high. The costs can be greatly reduced by using the TFT flat-panel image sensors.
The optical fingerprint sensor as commonly used consumer goods has requirements for lower cost and a thinner structure, thus the focus lens are not usually applied, while a flat-panel-like image sensor may be used instead. Therefore, an optical fingerprint sensor product which uses the TFT flat-panel image sensor has already been produced.
The TFT flat-panel image sensor is based on a substrate (e.g., glass, stainless steel, or plastic etc.), and a pixel array and a peripheral circuit may be manufactured through amorphous Silicon Thin Film Transistor (a-Si TFT), Low Temperature Poly Silicon Thin Film Transistor (LTPS TFT) or Oxide Semiconductor Thin Film Transistor (OSTFT) technology.
However, the TFT has a poor performance compared with the Si-based CMOS. Therefore, in the flat-panel image sensors produced by TFT technology, pixel signal amplification and digital-analog conversion function are usually achieved through an external chip, where the external chip is usually referred to as a signal readout chip, which may include a Readout Integrated Circuit (ROIC). That is, pixel electronic signals of the TFT flat-panel image sensor are connected to an external signal readout chip through data lines, and the signal readout chip performs an electronic conversion and signal processing so as to realize image capture. External laying ways of the signal readout chip can be mainly divided into three cases: in the first case, the signal readout chip is directly bonded to the TFT flat-panel image sensor in a Chip On Glass (COG) way; in the second case, the signal readout chip is directly bonded on a Flexible Printed Circuit (FPC) board, and then the FPC board is bonded to the TFT flat-panel image sensor; and in the third case, the signal readout chip is directly bonded on a rigid Printed Circuit Board (PCB), and then the rigid PCB is bonded to the TFT flat-panel image sensor via another FPC board.
FIG. 1 schematically illustrates a TFT flat-panel image sensor in a conventional technology. The TFT flat-panel image sensor may include a substrate 10, a signal readout chip (not shown), and a device layer (not shown) on the substrate 10. The device layer is manufactured on the substrate 10, and the substrate 10 may be made of glass, stainless steel, or plastic. The device layer may include a pixel array region and a peripheral circuit region, and may be manufactured through processes like amorphous silicon TFT technology, low-temperature poly silicon TFT technology, or oxide semiconductor TFT technology. The pixel array region has a plurality of data lines 11 and a plurality of scanning lines 12, the plurality of data lines 11 and the plurality of scanning lines 12 define a grid including a plurality of lattices arranged in array, and there is a pixel 13 in each lattice of the grid. The pixel 13 includes at least one pixel switch 131 (the pixel switch 131 is usually a TFT device), and at least one sensing device 132 (such as a photosensitive device, an electrode plate, or a thermosensitive device, etc.). The at least one sensing device 132 is applied to collect external input signals (such as light, electrostatic fields, heat, etc.), convert the external input signals into an electronic signal and then store the electronic signal in the pixel 13. When the at least one pixel switch 131 is turned on, the electrical signal in the at least one sensing device 132 is transmitted to the data line 11, and then the external signal readout chip performs signal capture. The plurality of scanning lines are controlled by a peripheral driving circuit to realize a row-by-row opening of the at least one pixel switch 131, where the peripheral driving circuit may be an external driving chip or be integrated into the device layer in a TFT device circuit form. Connection ways of the external driver chip can also be divided into three cases: in the first case, the external driver chip is directly bonded to a periphery of a TFT flat-panel image sensor; in the second case, the external driver chip is directly bonded on a flexible printed circuit board, and then the flexible printed circuit board is bonded to the TFT flat-panel image sensor; and in the third case, the external driver chip is directly bonded on a rigid printed circuit board, and then the rigid printed circuit board is bonded to the TFT flat-panel image sensor through another flexible printed circuit board.
Therefore, there is a need to improve a method for image capture of the TFT flat-panel image sensor in the conventional technology.